Reduce Ice-generated C++ warnings

Rather than simply listing (and ignoring) all the warnings
that come with ZeroC-generated code we now suppress their
output at the cmake level.

In addition, using `cmake -Dfatal-warnings=true .`
will make all warnings an error which forces cleaning
up these issues.

/cc @rleigh-dundee @sbesson -- suggestions on better header names welcome.
Neither of the above worked. I'm going to use something of the form:
```
+if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX)
+  set(ICE_FLAGS "-Wno-switch-default -Wno-redundant-decls -Wno-overloaded-virtual -Wno-old-style-cast -Wno-deprecated-declarations")
+endif()
```
for the moment, and if we have something more general, I'm happy to migrate.